A Via-First Approach to Fabricate Through-Silicon Vias Using Anisotropic Wet Etching of (100) Silicon Wafers

Through-silicon via (TSV) technology is one of the critical and enabling technologies for three-dimensional (3-D) chip stacking. In the roadmap for 3-D packaging, TSVs are mainly fabricated using deep reactive ion etching (DRIE) or a laser drilling approach. In this paper, we describe a generic method for fabrication of TSVs using wet etching of silicon, which can be applicable to many silicon-based technologies. This fabrication method is based on a via-first approach where the vias are fabricated prior to standard CMOS processing.

Conventionally, the DRIE process provides fine-pitch and high-aspect-ratio via holes, but the wet-etch process provides an economic alternative through batch processing and is easier for subsequent metallization due to a tapered via structure and a smooth via surface along the [111] crystalline plane. Traditionally, silicon nitride deposited using chemical vapor deposition has been used as an etch mask during the wet-etch process using potassium hydroxide (KOH) or tetramethylammonium hydroxide (TMAH). The silicon nitride etch mask presents several disadvantages, e.g., it forms pinholes during the etch process, has a high deposition temperature, and presents a higher cost.

Here, we present a spin-applied, photosensitive (negative acting) organic polymer material that could replace silicon nitride as a mask during wet-etch processes. The material is spin coated on the wafer, and a standard photolithography technique is used to create the via-hole pattern/image on the wafer. The wafer is subsequently immersed in an alkaline bath (KOH/TMAH) to etch silicon, thereby fabricating the tapered structure with the appropriate etch depth and via dimension.

Following this, the etch mask is removed and a temporary bonding material is coated on the same side of the wafer. Then, the silicon wafer is bonded to a perforated glass carrier. The bonded assembly undergoes back-grinding or is wet etched to open the vias from the backside of the wafer.

Once the vias are opened, the temporary bonding material is removed using a solvent to release the thinned wafer with TSVs from the glass carrier. A comparison of undercut between silicon nitride protection and polymeric mask protection was conducted to show that undercut for the polymeric mask is constant for stable etching conditions. Further wet-etch tests will be done at varying etch times and via dimensions for the fabrication of TSVs.